R-Value Drift

It is common knowledge in the building science
industry that all insulation systems perform at different levels depending
on environmental conditions. R-value performance can actually change as
much as 50% or more. Because Vermont, with its cold winters, is at the
extreme end of this variation in thermal performance, the impact of climate
cannot be ignored. While all insulation materials have published R-values
determined by laboratory testing, many of the materials we are using in our
buildings have published R-values that apply to other climates, not
Vermont. For example: if a particular insulation material or system performs
at R=19 in a moderate southern climate, but at 9.5 in a cold Vermont
climate, twice as much of that type of insulation should be used in Vermont
if the code requires a real-world performance equaling R-19.

The test results from the U.S. Department of Energy verify that Fiberglas R-values are compromised when a 25° Fahrenheit temperature difference exists between the inside (70°) and the outside (45°). When the test conditions reached an outside temperature of -18°, the R-values of the fiberglass insulation (both blown in and batts) decreased to less than 50% of the manufacturer’s stated R-value.

Cellulose R-values, on the other hand, went up. The R-value of Cellulose insulation began to rise when a 25° Fahrenheit temperature difference existed between the inside (70°) and the outside (45°). When the test conditions reached the outside temperature of -18°, the R-values of the Cellulose Insulation continued to rise above the manufacturer’s stated installed R-value. It was so determined that cellulose insulation improves in R-value any time a temperature difference of 25° or more exists

U.S. Department of Energy Test Results

Loose Fill Fiberglass

Metering Chamber (F)

Climate Temperature (F)

R-Value

70

44.6

17.8

70

32

16.1

70

26.8

14.1

70

8.6

12

70

-4.0

10.6

70

-18.4

9.2

Loose Fill Cellulose (Settled)

Metering Chamber (F)

Climate
Temperature (F)

R-Value

70

40

18

70

20

18.8

70

8

19.4

70

-5

19.6

70

-18

20.3

While this study only
looks at two of the many types of insulation on the
market, it is clear from this and a preponderance of
other test data that changes in temperature can
dramatically change the R-value of different
insulation products in very different ways.

Foam has low thermal drift

Architectural Testing Inc. of York, PA conducted similar tests in which the thermal performance of fiberglass was compared to that of polyurethane foam under comparable conditions. Report No: ATI-3822-5A conducted on 10/24/86 produced striking results.

Both products were tested in an average warm side ambient temperature of 68° F with a variety of cold side ambient temperatures and conditions. When the cold side temperature dropped to -15° F with a 15mph wind, the resulting thermal performances differed significantly. While the R-Value of the foam sample dropped to 12.0, the R-Value of the fiberglass sample dropped to 5.4. As conditions become more severe, the fiberglass performance became increasingly poorer. The integrity of the foam sample was far less compromised during harsh conditions.